This invention concerns a device for driving harnesses of weaving machines.
It is known that the shedding motion of weaving machines is obtained principally in three ways, namely by means of a cam drive, by means of a dobby or by means of a jacquard mechanism.
A cam drive is mainly used for a plain weave, dobbies are used for more intricate weaves such as checks, in which the harnesses can remain in the same position for a longer period of time, while jacquard mechanisms are used for the so-called jacquard textiles, in which as is known the warp threads are controlled separately. This invention concerns the first kind of drive.
In order to prevent large inertial forces, on high-speed weaving machines it is necessary that the harnesses be constructed as light as possible. It is also necessary for the harnesses to have as short a travel as possible, in other words to keep the amplitude of the up-and- down motion to a minimum. However, it is obvious that the motion of the harnesses depends on the preset opening angle of the shed. The harness situated the furthest from the fell line must carry out the largest motion, while the necessary motion becomes smaller as the harnesses are situated nearer to the fell line. A harness assembly with a large number of harnesses has to be kept as thin as possible, i.e. with the smallest possible distance between successive harnesses.
A known device for driving harnesses consists of levers connected to the harnesses and driven by means of negative cam drives. Such a negative cam drive means in this case that the motion of the levers is controlled in one sense by the cam and that the return motion is obtained by means of a spring which restores the lever to its original position. The use of such a negative cam drive offers the advantage that only one cam and cam follower are necessary per lever, so that in the case where the drives are provided directly underneath the harnesses, the width of the contact area of the cams and cam followers can be chosen to be almost as wide as half the distance between successive harnesses, i.e. half the pitch.
However, the above-mentioned negative cam drives are less suitable for high-speed weaving machines because the springs are not able to keep the cam followers in contact with the cams, unless very heavy constructions are used, which in turn require a large drive torque, even when the machine is running in slow motion.
In order to provide a solution for the above-mentioned disadvantage, devices are known in which the above-mentioned levers are driven by positive cam drives. Such a positive cam drive means that the oscillating movement of the levers is controlled in both senses by cams, so that return springs are no longer used. In such constructions, each lever carries two cam followers each of which operating in conjunction with a cam, known as conjugated cams, or "master and slave".
The use of a positive cam drive in turn causes problems. Indeed, for reasons stated above, it is necessary to keep the distance of the harnesses of high-speed weaving machines as small as possible, for example 12 mm. However, the use of a positive cam drive means that for each lever two cams must be provided next to one another, with the result that, also for structural reasons, the contact area of each cam can only be 3 to 4 mm wide. At very high rotation-speeds of the weaving machine, such contact areas are too narrow to support the strong inertial forces, causing the cams and cam followers to wear very quickly.
In order to solve this problem, it has already been suggested to make the body formed by the drive means wider than the harness assembly and to use bent transmission arms between the cam drives and the harnesses. However, such devices have the disadvantage that they are subjected to strong torsion moments, resulting in all kinds of negative effects. In order to limit wear phenomena, relatively expensive structures are necessary.
On the other hand it is also known to mount the two cams of each pair of cams belonging to one harness on different shafts, so that both cams are situated in one and the same area and both can make use of the entire width available. However, this solution has the disadvantage that many problems arise when setting the play and pretensioning of the conjugated cams, which must be carried out with the greatest precision. Such a construction is therefore not at all suitable for changing the crossing moment of the harnesses by turning the cams on their shafts.
In order to avoid the problem of synchronization of the conjugated cams, it is also known to make use of a negative cam drive, where the cams are mounted alternately on two different shafts. As a result, very wide cams and cam followers can be incorporated, as wide as 1.5 or 2 times the distance between the two harnesses. On the one hand such an embodiment allows a solid construction, yet on the other hand the above-mentioned disadvantage remains, i.e. the construction requires high drive torques, even when the machine is running in slow motion.